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You're on the right track; the applications that require public access should be installed and placed in the DMZ. I'm inferring from your question that you want to offer private applications to users on the internal network, but you want to do so using the same server that provides public applications. I do not recommend this approach for two reasons:
Mixing public and private services on the same server exposes you to significant application-layer risk. A flaw in the public application, such as an SQL injection vulnerability, could expose data used by the private application, and no additional layer of control or protection would be provided.
Services used solely by internal users do not belong in the same DMZ that provides services to the general public. The purpose of the DMZ is to provide a layer of isolation between public and private services. Even if you install the private application on a separate server within the DMZ, you still run the risk of compromising the server that runs the public processes. If that type of corruption occurs, you could potentially have a compromised server living in the same security zone as the private server. If both servers reside in the DMZ, traffic between them would not be regulated by the firewall.
My recommendation would be to implement a fourth firewall zone, using a separate network interface card (NIC) on the firewall or virtual LAN (vLAN) technology. This fourth zone -- perhaps call it the "internal DMZ" -- would be for services that process private data and should be restricted to internal users only.
More information:
Learn how VLANs can compartmentalize WLAN traffic.
A demilitarized zone protects systems from an affected server, but enterprise users should not be placed in the DMZ. Mike Chapple explains where they belong.
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